The present invention relates generally to a cascade multijunction solar cell construction.
Solar cells are photovoltaic devices which are capable of converting solar radiation into usable electrical energy. The energy conversion occurs as a result of the "photovoltaic effect". When solar radiation impinges on a solar cell, it is absorbed by the active region of the cell, causing electrons and holes to be generated. The electrons and holes are separated by the electric field resulting from the PN junction in the solar cell. The electric field is inherent in a semiconductor layer adjacent regions of P type, intrinsic, and N type GaAs semiconductors. Absorption of solar radiation of the appropriate wavelength produces electron-hole pairs in the collection region of the semiconductor layers. The separation of the electron-hole pairs, with the electrons flowing toward the region of N type conductivity, and the holes flowing toward the region of P type conductivity gives rise to the photovoltage and photocurrent of the cell.
The overall performance of the solar cell is maximized by increasing the total number of photons of differing energy which are absorbed by the semiconductor device. The task of expanding the range of the solar spectrum used by solar cells is alleviated, to some extent, by the systems of the following U.S. Patents, the disclosures of which are incorporated by reference:
U.S. Pat. No. 3,472,698 issued to Mandelkorn; PA1 U.S. Pat. No. 4,106,047 issued to Lindmayer; PA1 U.S. Pat. No. 4,289,920 issued to Hovel; PA1 U.S. Pat. No. 4,292,461 issued to Hovel; PA1 U.S. Pat. No. 4,316,049 issued to Hanak; and PA1 U.S. Pat. No. 4,332,974 issued to Fraas.
The above-cited references all disclose state-of-the-art solar cell constructions. Of particular note is the Hanak reference which discloses a high voltage series connected tandem junction solar battery having one optical path and is electrically interconnected by a tunnel junction. The layers of silicon arranged in tandem configuration can have either the same or differing bandgaps. A cell "interconnecting layer" is situated between the active semiconductor layers. It provides a single electrical path through the active layers to the back contact. The cell "interconnecting layer" also permits the transmission of solar radiation which is not absorbed by the active region to the second active region or additional active region, where additional absorption can occur. Tunnel junctions for amorphous-crystalline tandem solar cells are also disclosed in the Hovel Pat. No. 4,292,461.
Fraas is concerned with a multijunction solar cell and Hovel U.S. Pat. No. 4,289,920 is directed to a multiple bandgap solar cell. Lindmayer shows a solar cell with a discontinuous junction.
In Mandelkorn a cover glass is metallized in a pattern which is identical to the top contact pattern of a shallow junction solar cell. The cover glass is then bonded to the cell only within the metallized regions of the glass and cell.
The multijunction solar cell systems cited above depict attempts in the art to select a choice of bandgaps which allow the solar cell constructions to efficiently utilize greater ranges of the incident solar spectrum. While these systems are instructive, the prior art cascade solar cells have four constraints placed on the design of the junction between solar cells which are joined in an electrical and optical series; these are as follows:
First, a cascade solar cell construction entails a top cell, a tunnel junction layer, and a bottom cell. In such a construct, the first constraint is that the bandgap of the tunnel junction layer must be equal to or greater than the bandgap of the top cell (for transparency). In the systems of the above-cited references, there is no escape from the first constraint in cascade solar cell constructions.
The second design constraint is that the lattice constant of the tunnel junction layer must match that of the rest of the cell. This design constraint is typical of all semiconductor constructs.
The third design constraint on cascade solar cell construction is that the doping concentrations and profile gradients must be large enough to be consistent with the required peak tunneling current density. Note how this third constraint is in a potential conflict with the first design constraint on the bandgap of the tunnel junction layer.
The fourth design constraint on cascade solar cell constructions is that the crystalline perfection of the doped tunnel junction epilayers must be sufficient to allow high quality epitaxy of the top cell epilayers, while avoiding reductions in the top cell lifetimes and diffusion lengths due to propagating defects orginating in the tunnel junction layers.
As described above, the design of cascade solar cells systems entails orchestrating the four design constraints to optimize the efficiency of the solar cell performance. However, the design of cascade solar cell constructions could be enhanced if the design could be set free from one of the four major constraints. In response to this, the present invention includes a cascade solar cell design that does not require a transparent tunnel junction. This design effectively decouples one of the major constraints (bandgap) from the tunnel junction criteria and greatly increases the design flexibility of the cascade solar cell.